I can think of two reasons. One reason is that all these countries in their pre-modern histories had absolute monarchies, and these monarchies did not tolerate, in fact they could not tolerate, a loyal opposition. In an absolute monarchy, to be in opposition is to be in rebellion against sovereign authority. Colonialism wiped away the monarchies for the most part, certainly the absolute monarchies, but here and there we find a little monarchy left over in the postcolonial period: Cambodia; Malaysia; and of course Thailand, where the monarchy, with the aid of the military and the political astuteness of the monarch, demonstrated remarkable resilience and capacity to re-build itself from its dire circumstances in the mid-1930s and again in the late 1940s.